This invention relates to a pressure sensitive copying system and more particularly to pressure sensitive copying paper having a good color formability and an improved printability.
Usually the so-called "pressure sensitive copying system" consists of these three kinds of basic sheets such as top sheet, middle sheet and bottom sheet, wherein the top sheet is coated on the underside thereof with a composition consisting mainly of pressurerupturable microcapsules each enclosing an oily core material containing an electron donating organic chromogenic material (hereinafter referred to as "color former") dissolved or dispersed therein, the middle sheet is coated on the upperside thereof with another composition consisting mainly of electron accepting acidic reactant material (hereinafter referred to as "acceptor") which will produce a colored image when contact with the color former and also is coated on the underside thereof with the composition of microcapsules containing oil droplets in which a color former is dissolved or dispersed and the bottom sheet is coated on the upperside thereof with the composition of acceptor. One top sheet and one bottom sheet or, one top sheet, at least one middle sheet and one bottom sheet are superposed in that order to form a set of copying sheet in such a manner that the microcapsule coating layer and the acceptor coating layer are in contact with each other in each adjoining two sheets. Any partial pressing on the upperside of the top sheet of the thus prepared copying system with a pen or a typewriter will break the microcapsules positioned on the pressing, resulting in making the color former react with the acceptor so as to develop a color only on the part pressed.
In another pressure sensitive copying system, there are disposed on one surface of the same sheet both the acceptor and the microcapsules containing oil droplets in which the color former is dissolved or dispersed. The system is known as the "self contained" system.
The term "oily core material" described means the oil material which is enclosed in each color former microcapsule and comprises a non-volatile oil, a color former dissolved or dispersed in said non-volatile oil and other various additives which may be added when required. The color former microcapsules may be produced by any conventional method e.g., by the "coacervation" technique, by the "interfacial polymerization" technique or by the "in-situ polymerization" technique.
Among known color former compounds there are included triarylmethane compounds such as 3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (CVL), 3,3-bis(p-dimethylaminophenyl)phthalide, 3-(p-dimethylaminophenyl)-3-(1,2-dimethylindole-3-yl)phthalide and 3-(p-dimethylaminophenyl)-3-(2-methylindole-3-yl)phthalide, diphenylmethane compounds such as 4,4'-bis-dimethylaminobenzhydrylbenzylether, N-halophenyl-leucoauramine and N-2,4,5-trichlorophenyl-leucoauramine, xanthene compounds such as rhodamine-B-anilinolactam, rhodamine(p-nitroanilino)lactam, rhodamine(o-chloroanilino)lactam, 7-dimethylamino-2-methoxyfluoran and 7-diethylamino-2-methoxyfluoran, thiazine compounds such as benzoyl-leucomethyleneblue and p-nitrobenzoyl-leucomethyleneblue, spiro compounds such as 3-methyl-spiro-dinaphtopyran, 3-ethyl-spirodinaphthopyran, 3-phenyl-spiro-dinaphthopyran, 3-benzyl-spiro-dinaphthopyran, 3-methyl-naphtho-(6'-methoxybenzo)spiropyran, and 3-propyl-spiro-dibenzopyran.
Among known acceptors there are included inorganic acceptors such as acid clay, activated clay, attapulgite, zeolite, bentonite, silica and aluminum silicate; and organic acceptors such as phenol polymers, e.g., phenol-aldehyde polymers and phenol-acetylene polymers, aromatic carboxylic acids and polyvalent metal salts thereof.
Usually, the acceptor coated sheet is produced by coating a base sheet with a coating composition which comprises an aqueous dispersion of an acceptor and a binder. Among known binders there are included natural binder such as starch and casein; and synthetic binder such as polyvinylalcohol, carboxymethylcellulose, styrene-butadiene copolymer latex and acrylic acid copolymer latex.
The acceptor and the binder have antinomic functions with respect to the color formability and the printability. Namely, if a binder is used in a large amount in order to provide coating layer with a good surface strength so that "picking" at the time of printing can be prevented, the acceptor coated sheet shows a poor color formability. On the contrary, if the amount of a binder used is decreased to ensure a good color formability, the printing speed must be reduced in order to prevent "picking" during the printing operation.
An attempt has been made to improve the color formability of an inorganic acceptor coated sheet by utilizing in acceptor coating composition an aqueous emulsion of an oily material such as olive oil, fish oil, paraffin oil, chlorinated diphenyl, silicone oil, benzene and fluorinated and chlorinated ethylene, as disclosed in Japanese Patent Publication No. 13,165 of 1971.
Though the utilization of the aqueous emulsions of the kind disclosed therein can certainly improve the color formability of the acceptor coated sheet, it can not prevent the surface strength of the acceptor coated sheet from being decreased.
The principal object of the invention is to provide an improved acceptor coated sheet for a pressure sensitive copying system in which both the color formability and the printability are improved without sacrificing each other.
Another object of the invention is to provide an improved acceptor coating composition which includes a binder in a relatively small amount.
Other objects and advantages of the invention will be apparent from the following descriptions.